1. Field of the Invention
The invention relates to a marking substance for security elements used to examine the authenticity of documents such as bank notes and the like.
In order to raise the level of certainty against counterfeiting, marking substances are incorporated in paper pulp lines used in the manufacture of documents such as bank notes as well as of other products. Light-activated marking substances hitherto used are at present readily available in the market place, so that counterfeiters are in a position to falsify security elements fabricated with such marking substances.
2. The Prior Art
In this connection, EP 753 623 discloses a security sheet with an electrically conductive element. The security sheet consists of a carrier matrix of predetermined fiber structure, and its conductive element is a security thread constituted by a foil. The foil is coated with an electrically conductive polymer from the group of polythiophenes. The electrically conductive polymer is applied to the foil in liquid or dispersed form.
U.S. Pat. No. 5,112,672 discloses a security document with an imbedded electrically conductive security thread. The security thread is coated with a metal which in turn is provided with an electrically conductive polymer for bridging any interruptions.
U.S. Pat. No. 5,419,424 discloses a device for testing the security thread in bank notes. The testing device is provided with sensor electrodes which detect the security thread by capacitive coupling.
DE 43 34 797 discloses a method of fabricating counterfeit-proof documents as well as a method of testing their authenticity. The documents contain a grid work of metal wires which are contacted at their junctions.
In EP 839 950 there is disclosed a method of incorporating substances in a running fibrous web. The substances are introduced into the fibre suspension at or ahead of the site where the web enters into the machine. The substances are introduced at several sites distributed over the width of the fiber web, and at least one site the substances are introduced intermittently.
In order in such circumstances to put up barriers against counterfeiters complicated processes have been devised using light-activated marking substances in which, as described in German patent specification DE 196 53 423, light absorbing substances are additionally used which are invisible to the naked eye. In this manner printed images with noticeable error sections are generated when conducting tests under infra-red light.
Furthermore, to improve the safety against counterfeiting, marking substances are applied in a predetermined distribution to a web of paper to render their authenticity machine-readable. In accordance with DE 197 14 519 substances not visible by humans are used for this purpose, which are superimposed as linearly designed markings on a visible printed image. Because of its physical property the marking substance is supposedly detectable by a machine. Electrical conductivity is mentioned, among others, as one of the physical properties; however, there is no teaching of any marking substance which is invisible to the human eye.
A security element currently commonly used in bank notes is embodied by a foil structure consisting at least of a support foil and a metallization applied to the support foil. A so-called security thread is embedded, either completely or with windows (interruptions), into the paper web. Originally, such a security thread including recognizable demetallized sections shaped as symbols or letters served only for visual testing by humans. In attempting to improve the safety against counterfeiting, additional testing of the electrical conductivity of the metallization was being considered. However, until now, the realization of such attempts has been frustrated, on the one hand, not only by the high mechanical wear suffered by bank notes as a result, for instance, of creasing and folding by a user, but also by bending in automatic teller machines and counting machines. On the other hand, even during the technological process of manufacturing the paper the foil structure, as a result of tension and bending, is already subjected to considerable stress. There will thus occur randomly distributed fine hairline fissures in the metallization which render any test result uncertain and not reproducible. However, to counteract counterfeiting of these security elements, it is not only necessary to prove the presence of a metallization in bank notes, but authenticity must be recognized on the basis of measuring a certain conductivity value. In principle, this problem cannot be solved by the use of metallically acting printing inks instead of vapor deposited metallizations, as proposed by DE 43 44 553 and EP 0 659 587.
Since electrical conductivity is one of the most essential properties of metals, it seems to be obvious that counterfeiters will assume the electrical conductivity of a metallization. In fact, technological equipment is currently readily available for incorporating actual metallizations including their image-like designs as counterfeits of a security element into documents, securities, bank notes, wrappings or products. However, since electrical conductivity is a testing parameter which can be detected quickly and with certainty, no desire exists at present to do away with this security element. It is an additional disadvantage that the properties of the metallization which is visible to the human eye are substantially constant, as for the majority of users it is to serve as a constant security element always recognizable in the same manner. Finally, a relatively large number of persons is familiar with the secrets connected with the fabrication and testing of this humanly recognizable security element, so that the size and undefinability of this group of persons introduces a further risk potential.